This invention relates to eddy current brakes, and particularly to those which are used in the field of well drilling.
The use of the electrical eddy current brake as an auxiliary braking unit has come into general use by well drillers within the last twenty-five years. Such units are superior to water brake devices in that they provide high braking torque at rotational speeds and have no drag when not in operation. The eddy current brake units employed have been principally of the drum type which are directly attached to drill string drawworks. The rotor consists of an outer rotatable drum disposed about internal electromagnetic DC coils. The amount of torque produced depends upon the rotational speed of the drum and the magnetic flux density, and the electrical resistivity of the drum material in which eddy currents are induced. The descent of the drill string in the well is retarded by the resistance to rotation of the drum of the eddy current brake. The retarding force depends upon the speed of rotation of the drum and the strength of the magnetic field induced in the drum. The strength of the magnetic field is varied by a DC power supply which is controlled by the operator.
These units can handle a maximum retarding torque of approximately one hundred and fifty thousand foot pounds in the larger units. The size of such units is approximately six and one-half feet high and five feet in width.
In the course of the operation of these units a large amount of heat is generated. To preserve operable stability of the eddy current brake, it is necessary to carry off the heat generated during operation by passing cooling fluid through the unit. Water is generally used as the cooling liquid, and can be supplied at the rate of approximately one hundred and fifty gallons per minute for the larger type of eddy current units. Such water should be of low mineral content and of sufficient quantity and of low temperature to adequately carry off the heat generated.
Smaller sized units have been developed which would handle a maximum torque of approximately twenty thousand foot pounds. There are generally three to four different sizes of units offered by manufacturers for use. There is a need for a smaller more compact auxiliary braking unit which will have relatively high braking torque and will be more thermally efficiently cooled than the drum-type eddy current units now in general use.